Abstract: 40K is subjected to branched decay to form 40Ar by electron capture and 40Ca by beta (β-)decay.In general cases,the 40K-40Ar or 39Ar-40Ar and 40K-40Ca ages are consistent in the error range.In order to test the measuring system ,the K and Ca contents and their isotopic compositions in the potassium feldspar(W03116),pellet(W7200),biotite(ZBH-25),hornblende(ZBJ) national standards,and the Tachiti secondary international standard have been determined and their results are certified values in the error range.The 40K-40Ca isochron was made for 30 data of 40Ca／42Ca and 40K／42Ca in phengite, omphacite and garnet in eclogite from Bixiling,Dabie Mountains and showed a slope of S=0.1208±0.0423.corresponding to a 40K-40Ca age of 228±75Ma. This is close to the Rb-Sr age of 212 M a and Sm-Nd age of 239Ma in the eror range.However.the 39Ar-40Ar isochron ages of phengite and omphacite are 807.1± 32.6 Ma and 456.7± 27.2 Ma, respectively,which are obviously older than that of 40K-40Ca age.It provides a direct evidence for phengite and omphacite with excess argon due to both isotopic systems of K-Ar an d K -Ca belonging to the branched decays. In addition.the primary value(40 Ca／40Ca)i of the K-Ca isochron is equal to 151.424±0.013.and corresponds to Ca +27.Because the mantle rocks have Ca=0,it is certified that the original rocks of the eclogite have the continental character. This provides a new criterion that the ultrahigh-pressure metamorphic rocks at Bixiling in Dabie Mountains were formed during the process of subduction of the continental materials to the upper mantle and their exhumation back to the continent.